An airbag (air cushion) systems which are provided for the driver's seat and front passenger's seat for increasing the safety of automobiles are already becoming standard equipment. A conventional airbag system employs an explosive chemical to generate a gas to inflate an airbag. However, due to a desire for response in the speed of airbag inflation and control of the pressure of the inflating gas, another type of airbag system, which stores a high pressure gas for inflating an airbag in a steel pipe referred to as an inflator (or an accumulator), has been developed and is being increasingly employed.
A typical inflator has a structure in which a steel pipe having both ends reduced by necking is closed at one end by welding a cover plate thereto and has an airbag activating mechanism mounted at the other end. When a collision is sensed, the high pressure gas stored in the inflator is discharged all at once into the airbag. Accordingly, a steel pipe for an airbag inflator undergoes stress in an extremely short length of time at a large strain rate. Therefore, in contrast to conventional structural members such as pressure cylinders and line pipes, a steel pipe for an airbag inflator is required to have high strength, excellent resistance to bursting, and good toughness, in addition to high dimensional accuracy and good workability and weldability. Since automobiles are used even in frigid climates, the resistance to bursting should be guaranteed at a temperature as low as −40° C. or lower.
A steel pipe for an airbag which is suitable for use as an inflator and a process for its manufacture are described in JP-A 10-140238, JP-A 2002-194501, JP-A 2002-294339, and JP-A 2003-171738, for example. However, although the technology described in these patent publications aims at providing a steel pipe for an airbag inflator having high strength and high toughness, the tensile strength achieved therein is on the 800 MPa level or 900 MPa level and is at most 947 MPa, and there are no pipes which realize a high strength at the level of at least 1000 MPa along with excellent toughness.